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Last Updated: Monday, 22 September, 2003, 16:16 GMT 17:16 UK
Long wait for Jupiter return
By Dr David Whitehouse
BBC News Online science editor

Despite Galileo's stunning success, and the scientific treasures it discovered at Jupiter, it will be many years before we return to the planet.

Jupiter Icy Moons Orbiter
We will have to wait until 2018
Only the distant gaze of the Hubble Space Telescope will keep watch on the giant world's gas clouds, and the eruptions on its moon Io, the most volcanic body in our Solar System.

But of Galileo's major discovery - the properties of Jupiter's ice-crusted moons - except for a passing probe en route to Pluto in 2007 - we will have to wait a long time to investigate them up close again.

In 2018 in fact, by which time the Galileo mission will be scientific history. The probe itself will be thin smears of metal dispersed in the lower atmosphere of the gas-giant.

Sister Saturn

After Galileo, the focus is turning to Jupiter's sister planet, Saturn.

Starting in 2004, the Cassini craft will orbit the ringed world for many years and drop a probe on to its major moon, Titan.

Cassini Saturn mission
Cassini: Arriving in 2004
Titan, with its thick, chemically rich atmosphere, will be especially interesting to scientists.

Some believe that before life arose on the primitive Earth, our world was in a similar state, and that the secrets of life's origins may be revealed in this far-away moon's murk.

Then our attention will turn towards Mars.

Every two years, a probe - sometimes more than one - will be dispatched to the Red Planet. At first, these missions will reconnoitre the Martian surface, then return rock samples, and then perhaps establish an automated laboratory on the surface.

Another Jupiter mission has to wait its turn.

High priority

At least three of Jupiter's icy moons - Callisto, Ganymede and Europa - have subsurface oceans. On Europa, there is evidence that liquid water has been in contact with the surface in the recent past.

Ganymede and Europa, Nasa
Icy moons are a future focus
These subsurface oceans are Galileo's major discovery; indeed they are one of the major discoveries of the space age.

Just before the end of the Galileo mission, the US National Research Council ranked Jupiter's moon Europa as one of its highest priorities in planetary exploration.

Although Galileo's follow-on was conceived before the probe's demise, the pace of development means that the next craft will not arrive at the giant planet for another 15 years.

New power

The new mission, Jimo - the Jupiter Icy Moons Orbiter - will use an ion-drive engine.

These engines shoot out a propellant gas much faster than a chemical rocket. Their greater power will enable Jimo to go into orbit around Jupiter's moons rather than just conduct brief flybys like Galileo.

Ion drives have been tried several times over the years. Test rigs were on a Soviet space station and an early version of the technology has even flown on the Deep Space 1 probe. Europe will also trial an ion-drive technology on its Smart-1 Moon probe this month.

But Jimo's ion-drive will be in a different league. Powered by a small nuclear reactor, it will have a hundred times the efficiency of a chemical rocket.

As it leaves Earth orbit, Jimo will unfurl a series of heat-radiating panels designed to prevent the spacecraft from overheating.

Then it will begin the long haul to Jupiter. It will take six years.

Encounter at last

Shortly after entering Jovian orbit in 2018, Jimo's trajectory will take it close to Callisto.

Largest moon in the Solar System, Nasa
Ganymede: A transitory world
Callisto, outermost of the so-called Galilean moons, is only slightly smaller than the planet Mercury. It has what may be the oldest cratered surface in the Solar System. There is much history to be read there.

Five months later, a prolonged burn of Jimo's ion engine will liberate it from Callisto's gravitational grip. As it loses orbital energy, Jimo will fall towards Jupiter, to encounter its next target, Ganymede.

Ganymede is the largest of Jupiter's satellites. Indeed, it is the largest moon in the Solar System. It has a curious mixture of old and young surface regions. It is a transitory world.

Leaving Ganymede, Jimo's final port of call will be Europa - the highlight of the mission.

Ice drill

Europa is in a more intense part of Jupiter's magnetosheath and the radiation is extreme. Despite precautions such as the radiation "hardening" of its electronics, Jimo cannot be expected to operate for very long around Europa.

Jupiter's moon Europa, Nasa
Europa could harbour life
Jimo will carry a radar for mapping and determining the thickness of the ice crust, a laser altimeter to produce surface profiles, cameras and devices to measure magnetic fields as well as gas and dust.

But all too soon Jimo is expected to be crippled beyond repair by the radiation. It, too, will be disposed of in Jupiter's atmosphere.

Hopefully, however, the next mission should be well advanced by then and ready to take its place.

This would have the capability to land on Europa's icy plains and melt a small sample of ice to look for signs of life.

Galileo ends in blaze of glory
21 Sep 03  |  Science/Nature
In pictures: Galileo's voyage
21 Sep 03  |  Photo Gallery
Galileo: Space success story
20 Sep 03  |  Science/Nature
Europa's ice crust probed
18 May 03  |  Science/Nature

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